Along with the trend of wafer thinning, wafer warpage and deformation has reached millimeter level. However, when warpage volume increases but the focus range is not large enough and the focusing speed is not fast enough, the focusing of the automatic optical inspection (AOI) system will become more difficult and the detecting time will be prolonged. Consequently, the AOI system will have poor performance in detection, and subsequent manufacturing processes will be affected.
Of the current technologies, the focusing mode mainly includes an active focus mode and a passive focus mode. The focusing process of the passive focus mode includes a coarse tuning stage and a fine tuning stage. In the active focus mode, the light generated by a light source is projected onto a focusing grating and then is further projected on the surface of an object via a lens. A phase comparison between the grating image reflected from the surface of the object and the original grating image is made to increase the focusing speed. However, the active focus mode incurs a higher cost.